1. Field of the Invention
The present invention relates to an electric double-layer capacitor and more particularly, to an electric double-layer capacitor having positive and negative polarizable electrodes each of which is made of a solid activated-carbon, and a fabrication method of the capacitor.
2. Description of the Prior Art
An electric double-layer capacitor is a capacitor utilizing an electric double-layer generated at an interface of a solid (i.e., a polarizable electrode) and an electrolyte solution. This capacitor has a feature that a large capacitance in the order of farad (F) is readily realized, due to the fact that the electric double-layer equivalent to a dielectric layer in a popular capacitor is approximately as small as a molecule diameter.
FIGS. 1, 2 and 3 show a conventional electric double-layer capacitor, which was disclosed in the NEC Technical Journal, Vol. 47, No. 10, pp. 91-97, issued by the NEC Corporation on Oct. 11, 1994.
As shown in FIG. 1, this conventional electric double-layer capacitor has a stacked structure comprised of four basic cells 130. The cells 130 are stacked in a direction perpendicular to the cells 130, and are electrically connected in series. Each of the basic cells 130 is a unit of stacking.
As shown in FIG. 1, each of the basic cells 130 is comprised of a pair of polarizable electrodes 101 onto which an electrolyte solution such as a water solution of sulfuric acid is absorbed, a non-electron-conductive, porous sheet-like separator 102 sandwiched between the pair of electrodes 101, an insulating tubular gasket 103 having a tubular inner space 103a in which the pair of electrodes 101 and the separator 102 are placed, and a pair of conductive sheet-like collectors 108 located on both sides of the gasket 103 to close its open ends, respectively.
As shown in FIG. 3, the gasket 103 has an opening 104 for supplying the electrolyte solution into the inside of the basic cell 130. The opening 104 is formed to penetrate the gasket 103 in a direction parallel to the gasket 103, and is sealed with a plug 118. The gasket 103 further has four circular holes 106 at its respective corners for inserting connection bolts 116 thereinto.
The pair of polarizable electrodes 101 are typically made of a solid activated-carbon. The pair of collectors 108 are made of, for example, a conductive rubber or plastic, the conductivity of which is achieved by mixing carbon powder into the rubber or plastic material. The separator 102 is made of, for example, a porous polyolefin-system plastic or glass fibers, and is non-electron-conductive and ion-permeable.
The pair of collectors 108 serve not only as terminal plates of the basic cell 130 but also as sealing members for the electrolyte solution together with the gasket 103.
The collectors 108 located between the adjacent two electrodes 101 are commonly used for the adjacent two basic cells 130, respectively.
A pair of external terminal plates 110 are attached onto the outermost collectors 108 of the stacked structure formed by the four basic cells 130, respectively. A pair of rubber plates 112 serving as spacers are attached onto the pair of external terminal plates 110, respectively. A pair of pressing plates 113 are attached onto the pair of rubber plates 112, respectively.
The connection bolts 116, which are made of a metal such as a stainless steel, are inserted into the stacked holes of the stacked gaskets 103, respectively. Nuts 117, which are made of a metal such as a stainless steel, are engaged with the screws formed at the both ends of the bolts 116 to press the stacked basic cells 130 along the rods 116, thereby holding or combining the four stacked basic cells 130 together.
Due to the applied pressure, the contact resistance across the adjacent basic cells 130 and that across the outermost basic cells 130 and the corresponding terminal plates 110 are kept to a mimimum.
Generally, the above basic cell 130 independently exhibits a charge-storage function, and as a result, the single cell 130 maybe used as an electric double-layer capacitor. In actuality, however, a plurality of the basic cells 130 are often connected in cascade to thereby constitute the stacked structure, as shown in FIG. 1. The purpose of this stacked structure is to provide sufficient dielectric strength against the supply voltage for the electronic circuit in which the electric double-layer capacitor is used.
Specifically, the dielectric strength for the basic cell 130, which serves as an electric double-layer capacitor of the single-cell structure, depends upon the electrolysis voltage for the solvent of the electrolyte solution. For example, with an electric double-layer capacitor using a water-soluble electrolyte such as a dilute water solution of sulfuric acid, the dielectric strength is approximately 1.0 V, which is equal to the electrolysis voltage for water. The dielectric strength as low as approximately 1.0 V is insufficient against the supply voltage of 5.0 V, for example, that is typically used for a semiconductor integrated circuit. Therefore, in this case, at least six basic cells 130 are connected in cascade to thereby increase the dielectric strength.
Next, the fabrication method for the electric double-layer capacitor as shown in FIG. 1 is explained below with reference to FIGS. 2 and 3.
FIG. 2 shows a partial cross-section of the electric double-layer capacitor as shown in FIG. 1 in the boundary area between an adjacent two of the basic cells 130. FIG. 3 shows a cross-sectional view along the line III--III in FIG. 2.
First, the polarizable electrodes 101 are prepared by a known method. As an example, the fabrication method of the activated-carbon/polyacen composite material as disclosed in the Non-Examined Patent Publication No. 4-288361 published in 1992 may be used. Also, the collectors 108 are prepared by a known method.
Then, as shown in FIG. 2, the two electrodes 101 are pressed and attached onto the both surfaces of each of collectors 108, respectively. Thus, one of the collectors 108 and two of the electrodes 101 are unified or combined together.
The porous separators 102 and the gasket 103 are prepared by know processes, respectively.
Next, the separator 102 is placed on the electrode 101 in the subassembly consisting of the collector 108 and the two electrodes 101, and then, the gasket 103 is placed around the subassembly, as shown in FIG. 2.
Further, another subassembly consisting of the collector 108 and the two electrodes 101 is placed on the underlying separator 108. Thus, this cycle of operations is repeated as necessary, thereby forming the stacked structure.
Subsequently, as shown in FIG. 1, the terminal plates 110, the rubber plates 112, and the pressing plates 113 for uniform application of the pressure are applied to both ends of the stacked structure. Using the bolts 116 and nuts 117, the assembly thus formed is then held together under application of pressure.
Finally, the dilute sulfuric acid as the electrolyte solution is poured into the inside of the basic cells 130 through the corresponding openings 104 of the respective gaskets 103, thereby impregnating the activated-carbon electrodes 101 with the electrolyte solution by an impregnation method such as the vacuum impregnation method. Then, the openings 104 are plugged with the sealing plugs 118. Thus, the electric double-layer capacitor of the stacked structure shown in FIG. 1 is finished.
As seen from the above description, with the conventional electric double-layer capacitor of FIG. 1, the basic cell 130 is the unit of this capacitor from the viewpoint of charge storage function. On the other hand, from the viewpoint of fabrication method, a unit cell shown in FIG. 2, which is comprised of the collector 108 and the two electrodes 101 attached onto the corresponding surfaces of the collector 108, is a unit of this capacitor.
In other words, the functional repetition unit 130 and the fabricating repetition unit 131 are displaced from each other. Hereafter, the functional repetition unit may be called the "basic cell", or as necessary, the "functional basic cell", and the fabricating repetition unit may be called the "unit cell", or as needed, the "fabricating unit cell".
With the conventional electric double-layer capacitor shown in FIGS. 1, 2, and 3, the components required for fabrication are separately and individually provided, except for the subassembly comprised of the collector 108 and the two electrodes 101 that are previously integrated or unified. The problem with such conventional capacitors is that many assembly steps are required for the stacking process.
To solve the problem relating to the assembly processes, an improved technique was developed and disclosed in the Japanese Examined utility-Model Publication No. 7-31535 published in 1995. In this technique, the gasket of the electric double-layer capacitor is formed by injection molding of a thermoplastic resin. Further, the collector and the gasket are previously integrated or unified together during the injection-molding process for the gasket. Thus, the bonding strength is enhanced and the assembling operation is simplified.
With the conventional electric double-layer capacitor as shown in FIG. 1, no venting means is provided for any of the "functional electric basic cells 130". Therefore, there is the possibility that some cause such as generation of gas due to the electrolysis of the electrolyte solvent increases the internal pressure within the basic cells 130, thereby degrading the Equivalent Series Resistance (ESR). As a result, the basic cells 130 may burst, thereby destroying the charge storage capability as a capacitor. In a worst case scenario, the electrolyte solution is leaked from the basic cells 130 to thereby damage other electronic components.
To solve these problems, taking measures such measure as providing a venting valve in the "functional basic cell 130" is effective. An example of the electric double-layer capacitors having such the venting means was disclosed in the Japanese Non-Examined Utility-Model Publication No. 4-72617 published in 1992.
With the conventional electric double-layer capacitor as disclosed in the Japanese Non-Examined Utility-Model Publication No. 4-72617, a venting valve is provided for at least one of the stacked basic cells, and a water-repellingporous film, which does not pass the electrolyte solution but allows the gas existing in the cells to permeate through the film, is provided in a part of the intermediate collectors located between the adjacent basic cells. The gas generated in any one of the basic cells is released to the outside through the water-repelling porous film.
As described above, the conventional electric double-layer capacitor as shown in FIGS. 1, 2,and 3 has the problem that the components constituting the capacitor are each separate, and a lot of assembly processes are required to make the stacked structure. To solve this problem, the conventional electric double-layer capacitor disclosed in the Japanese Examined Utility-Model Publication No. 7-72617, provides a gasket integrally molded around the collector during the injection molding process of the thermoplastic resin, thereby decreasing the number of the necessary components. However, in this case, only two components, i.e., the collector and the gasket are integrated into one, and consequently, the effects of the reduction of the number of components and the simplification of the assembling operation cannot be said to be sufficient.
Further, the electric double-layer capacitor is also required to have a high reliability and a high safety as any other electronic components. Thus, with the conventional electric double-layer capacitor as disclosed in the Japanese Examined Utility-Model Publication No. 4-72617, a venting valve is provided for at least one of the stacked basic cells, and a water-repelling porous film is provided in a part of the intermediate collector located between basic cells, thereby allowing the gas generated in the cells to be released to the outside.
However, with the conventional capacitor disclosed in the Japanese Examined Utility-Model Publication No. 4-72617, because the direct connection between the inside of the basic cell and the outside thereof is realized by the venting valve, not only the gas but also the electrolyte solution tends to leak from the inside of the cells through this venting valve.
Fluoroplastics such as the polytetrafluoroethylene, which is the material to be used for the venting valve, are difficult to adhere to other engineering plastics. It is thus, a problem that the venting valve lacks reliability in such a configuration.